1. Field of the Disclosure
This disclosure relates to the field of DSL (Digital Subscriber Line) communications, and in particular, a method and apparatus to detect, analyze, and report DSL wiring defect conditions.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication. In particular, asymmetric DSL (ADSL) and very-high-speed DSL (VDSL) can adjust to the characteristics of the subscriber line by using a discrete multi-tone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line. DSL systems can use vectoring technologies, where joint transmitter and/or joint receiver signal processing can be performed among multiple pairs to mitigate the effects of crosstalk interference, and thus to improve performance.
The performance of DSL systems can be affected when there are undesired noise sources or when the loops are impaired. DSL systems would further benefit from determining the specific cause of a problem such as a DSL link instability and/or poor link quality that can lead to a DSL link failure, link error or loss of bandwidth and taking measures to report such problem and its corresponding cause, in order to get the problem fixed.
In particular, when there is a wiring problem or loop impairment near customer premise equipment, the downstream bit distribution can be distorted due to a variety of causes. One of such causes can be an incomplete common-mode rejection combined with noise harmonics from nearby electronics or other noise sources. This can happen if one of the two copper wires is impaired or when the impedance of the two wires is not matched well, hence resulting in an unbalanced line. An unbalance issue in the two wires makes them susceptible to common-mode noise. The source of the noise could be radiation from common radio sources in the DSL environment or inside DSL user homes. For example some lines are affected by noise sources with 40˜80 Khz harmonics, which are often generated by TV and computer monitors, such as HDTV sources. This noise can be measured for instance by using a spectrum analyzer by placing a probe near a noise source such as a laptop's screen, which could exhibit near 60 Khz harmonics.
Another possible cause is the short bounces of the DSL signal in the presence of wiring problems. Multi-port modeling with twisted pairs shows that cross-pair modes may vary by as much as 40 dB from “tone to tone” in extreme cases. The effect is magnified by larger imperfection in the twisting of the two wires (imperfection in twisting has similar effects as imperfection in balance). Indeed, for bad balance, such as the one occurring in the presence of the 3rd “wire” used in old telephones as the circuit for the bell, the third wire can be considered as an earth ground and the bad balance implies similar impact as irregular twisting. Basic multi-port transmission line theory models a transmission line by a series of incremental uniform segments. The discontinuities caused by the imperfections create multiple reflection points along the line leading to short bounces back-and-forth of the electromagnetic waves, which can cause the rapid notching in bit-distribution. In this case, the channel parameters such as Hlog or Hlin might show the notching.
In addition, DSL system loading and bit-swapping algorithms can be another source of the problem. When there is a wiring problem near customer premise equipment and the noise from electronics or other sources affect the line's bit loading, the noise spectrum received by the equipment can vary quickly in time because of the nature of noise sources. When the noise spectrum is varying quickly in time, a bit loading algorithm in the customer premises equipment might not be able to respond properly, hence resulting in an abnormal bit distribution. This especially can be the case, when the bit loading pattern does not match any frequency harmonics.
There are many known types of wiring problems in DSL systems. For example in some countries the in-house DSL wiring often includes a redundant third wire that was used for ringing a telephone bell several decades ago. The third-wire is not used any more, but the existence of such third wire in DSL systems creates an unbalanced impedance. The presence of a third wire results in a line imbalance, which in turn makes the system susceptible to external noise, signal bouncing, undesired bit loading, etc. as discussed above.
Moreover, other impairments (echo, external noise and serial-resistance/shunt/half-cut to name a few) could also cause instability in the line. In addition, the instability could appear in other line parameters such as the Hlog, noise or signal to noise (SNR) measurements. For example a serial-resistance/shunt/half-cut creates similar behavior in Hlog as well as bit loadings.
The DSL system operator and the customer would greatly benefit from detection of such wiring defect problems and their cause by evaluating the relevant data from the DSL system.
Existing line testing techniques often fail to identify the source of such impairments, and also require special devices and testing equipment. Many line testing techniques, involve using line probes, test signals, test equipment and devices, which measure physical characteristics of the line, or transmit signals on the lines, and measure reflections to find out information about the state of the line being tested. Most of these techniques require dispatching a technician to the customer site, which not only entails a large expense for the service providers, the testing procedure is also disruptive to the operation of the line. In these techniques, in order to perform the line testing, the normal operation of the line needs to be disrupted, while the testing and signal measurements are performed. In some cases, the service providers even seek the assistance of the customers. In such cases the customers are asked to manipulate in-house equipment, based on which the service provider will perform testing measurement to identify the wiring problem.
Embodiments of the current invention avoid all the above issues, by not requiring any testing devices, test signals, or disruptive measurements. The embodiments use the existing information collected from the line, without causing any disruption to the service.
The DSL system operator and the customer would greatly benefit from means for determining such wiring defect problems and their cause by evaluating the relevant data from the DSL system.